Superplasticity of certain metals at critical elevated temperatures, such as titanium and certain of its alloys, has been known for many years. The advantages of the superplastic characteristic is that the capability of extreme elongation of such material can be applied to the production of complex shapes. It is also known that at proper temperature and pressure certain metals can be diffusion bonded in which a homogeneous connection between two parts is obtained at the contacting interface or faying surfaces. Diffusion bonding in the case of titanium must take place in an inert atmosphere for protection of the surfaces to avoid oxidation contamination.
The prior art relating to superplastic forming is exemplified by U.S. Pat. to Fields, Jr. et al Nos. 3,340,101; and Hamilton et al 3,920,175 and 3,927,817. These patents have disclosed methods of superplastic forming and diffusion bonding. Generally the disclosure is limited to either simultaneous forming and bonding, or to the use of maskants to areas not desired to be bonded. The prior art appears suitable for producing flat structures of generally small plan form areas. However, the prior art has not disclosed any method suitable for obtaining in large area structures or structures having compound curvature characteristics, the uniform intimate contact of faying surfaces required for diffusion bonding. What has been made known from the past has not been able to solve the problems of obtaining accurate contour matching of the forming dies, of compensating for thermal and mechanical distortion of the dies or variations in stock material thickness and of affording control over the areas or locations where diffusion bonding is to take place without the use of maskants. Even when maskants must be used the control of the effectiveness or the location of the maskants is uncertain, particularly with compound curved parts.